Among the battery which is practically used, the lithium ion secondary battery exhibits the highest energy density, thus it is particularly used for the compact electronics. Also, in addition to the compact use, it is also expected to be used for the automobiles. Under such situation, it is in demand to make the lifetime of the lithium ion secondary battery longer, and to further improve the safety.
In the lithium ion secondary battery, generally in order to prevent the short circuit between the positive electrode and the negative electrode, polyolefin based organic separator such as polyethylene or polypropylene or so is used. The polyolefin based organic separator has a physical property which dissolves at 200° C. or less, thus in case the temperature of the inside of the battery becomes high due to the stimulus from the outside and/or the inside, the organic separator shrinks or melts, thereby the volume of the organic separator changes. As a result, the short circuit of the positive electrode and the negative electrode, and the explosion due to the release of the electrical energy or so may happen.
In order to solve such problems caused by using the polyolefin based organic separator, on the polyolefin based organic separator or on the electrodes (the positive electrode or the negative electrode), it is proposed to stack the layer (the porous membrane) comprising the non-conductive particle such as inorganic particle or so. Further, in order to prevent the overheat caused by abnormal reaction of the battery, the porous membrane including the polymer particle which melts by heat, or the polymer particle of which the swelling degree to the electrolytic solution increases by heat is proposed. In case the temperature of the secondary battery increases due to the short circuit or so, the fine air hole in the porous membrane is sealed by the polymer particle by melting or swelling, thereby the ion pass between the electrodes is interfered and the electrical current is blocked. Thus, it is thought that it has function to suppress the temperature from rising any further (the shutdown function).
For example, in the Patent document 1, the porous membrane using the inorganic particle being surface treated by the silane coupling agent and using SBR (styrene butadiene rubber) as the binder is described. Also, the Patent document 2 describes the porous membrane wherein the inorganic particle being surface treated with the coupling agent is used and PVD (polyvinyl butyral) as the binder in order to reduce the water amount of the porous membrane. Also, the Patent document 3 describes the method of improving the powder fall off of the non-conductive particle from the porous membrane by using the water dispersed acrylic polymer particle having the hydrophilic group such as sulfonic acid or so as the binder. Furthermore, the Patent document 3 describes that the porous membrane having strength and flexibility can be obtained by comprising the crosslinking group in the water dispersed acrylic polymer particle.